Field
This disclosure relates to generally to wireless communication devices, and more particularly to antennas for wireless access points including Multiple-Input, Multiple-Output (MIMO) radios.
Description of the Related Art
Smart phones, tablet computers, and other wireless communication devices are widely used for data networking. Data networks that use WiFi® (“Wireless Fidelity”), also known as “Wi-Fi,” are relatively easy to install, convenient to use, and supported by the Institute of Electrical and Electronic Engineers (IEEE) standard 802.11. The performance of WiFi data networks makes WiFi a suitable alternative to a wired data network for many business and home users.
WiFi networks operate by employing wireless access points that connect user devices (or client devices) in proximity to the access point to varying types of data networks such as, for example, an Ethernet network or the Internet. A wireless access point includes at least one radio that operates according to one or more of the standards specified in different sections of the IEEE 802.11 standard. Wireless access points may include omni-directional antennas that allow the radios within the access point to communicate with client devices in any direction. Alternatively, wireless access points may include directional antennas that allow each radio to communicate with client devices within a respective portion, or sector, of the area surrounding the access point. In this case, the sectors served by the individual radios may be distinct or may overlap. Each wireless access point is also connected to a data network such as the Internet through a backhaul communications link. The backhaul communication link is typically a hard-wired communication path such as an ethernet lick or a fiber optic link, but may also be a wireless communication path. User devices communicate with the data network via the wireless access point and the backhaul communications link.
The IEEE standards that define the radio configurations include:                A. IEEE 802.11a, which operates on the 5 GHz frequency band with data rates of up to 54 Mbs;        B. IEEE 802.11 b, which operates on the 2.4 GHz frequency band with data rates of up to 11 Mbs; and        C. IEEE 802.11g, which operates on the 2.4 GHz frequency band with data rates of up to 54 Mbs.        D. IEEE 802.11n, which operates on either the 2.4 GHz frequency band or the 5 GHz frequency band with increased data rates due to the use of multiple input/multiple output (MIMO) radios.        E. IEEE 802.11ac, which operates on the 5 GHz frequency band using MIMO radios with higher data rates than 802.11n.        
Both the 2.4 GHz and 5 GHz frequency bands are divided into multiple frequency channels. For example, the 2.4 GHz band is divided into 14 defined frequency channels. Not all countries allow the use of all defined channels. Further, the frequency spacing between adjacent channels in the 2.4 MHz band is only 5 MHz, which is smaller than the bandwidth required for WiFi communications. Thus only three or four non-overlapping channels are typically used at any particular location.
The use of MIMO radios in IEEE Standard 802.11n and 802.11ac results in more stable connections and higher data rates at the expense of requiring multiple antennas for reception and transmission of multiple streams at each radio. Within this patent, the term “channel” means a subdivision of a frequency band, and the term “stream” means the bidirectional signal flow between a radio and an antenna. IEEE 802.11n provides for up to four streams/antennas per radio. IEEE 802.11ac allows up to eight streams/antennas per radio. Commonly, a MIMO radio is defined by a descriptor “R×T”, where R is the number of received streams and T is the number of transmitted streams. The number of antennas is equal to the greater of R and T. For example, a “4×4” MIMO radio transmits and receives four streams using four antennas. The need for multiple antennas for MIMO radios complicates the physical design of access points, particularly when the access points include multiple radios.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.